Disk drives are digital data storage devices that can store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive includes a plurality of magnetic recording disks that are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads is disposed adjacent to surfaces of the disks to transfer data between the disks and a host computer. The transducers can be radially positioned over the disks by a rotary actuator and a closed loop, digital servo system, and can fly proximate the surfaces of the disks upon air bearings.
A plurality of nominally concentric tracks can be defined on each disk surface. A preamp and driver circuit generates write currents that are used by the transducer to selectively magnetize the tracks during a data write operation and amplifies read signals detected by the transducer from the selective magnetization of the tracks during a data read operation. A read/write channel and interface circuit are connected to the preamp and driver circuit to transfer the data between the disks and the host computer.
During a read operation, analog electric signals are induced in a read transducer as it travels above the surface of the disk. These input signals are filtered and amplified by an analog front end (AFE) circuit in the read/write channel. Information in the input signals is extracted by the read/write channel and provided to a data controller. In addition to containing information relating to the data stored on the disk, the input signals may also contain information relating to the operation of the disk drive. For example, changes in the flying height of the read/write head may cause characteristic changes in the waveform of the input signals. Thus, by examining certain characteristics of the input signals, a disk drive may determine if a read/write head is flying too low or too high.
For head flying height measurements, the input signal is the readback signal from the head for a known magnetic pattern written in the media. An example of the input signal is the single tone pattern, which includes a fixed number of magnetizations in one direction, followed by another fixed number of magnetization in the other direction. The resultant readback signal can be decomposed to the summation of
(1) a constant, also known as the DC content;
(2) a sinusoidal signal with the same frequency of the magnetization pattern, also known as the fundamental frequency component; and
(3) sinusoidal signals whose frequencies are the multiples of the fundamental frequency, also known as the harmonics.
By measuring the signal components at the read head output, the response of the head/media at the particular frequencies may be determined. A typical input signal has the same number of the magnetizations in each direction. There are no DC contents or harmonics whose frequencies are even multiples of the fundamental frequency.
Changes in the flying height of a read/write head may be manifested in the time domain by a change in the pulse width of an input signal. For example, the pulse width may be relatively narrow when the read/write head is flying close to the disk surface, and the pulse width may be relatively wide when the read/write head is flying farther away from the disk surface.
Analysis of the input signal waveform may be more conveniently and/or accurately performed in the frequency domain. In particular, the ratio of the first and third harmonic components of the input signal may be used to determine the flying height of a read/write head. The magnitudes of the first and third harmonic components of the input signal may be measured, for example, using a harmonic sensor at the output of the AFE circuit. However, since the input signal is filtered and amplified by the AFE circuit, the magnitude measurements may be affected by changes in the gain of the AFE circuit. Thus, changes in the magnitude measurements may be a result of changes in the flying height of the read/write head or by changes in the gain of the AFE circuit. The gain of the analog front end circuit may be affected by environmental conditions, such as variations in environmental temperature and/or changes in supply voltage, which may be uncorrelated to changes in the underlying input signal.
Calibration of the DC gain of an analog front end may be performed using an environmentally stable reference voltage, such as a bandgap reference voltage. However, the DC gain of the analog front end may respond to environmental changes differently from the gain of the analog front end at frequencies of interest.
Calibration of the AC gain of an analog front end may be performed using an internal calibration square wave signal having an amplitude set with reference to an internal bandgap reference voltage. However, calibration using a square wave signal may have a number of drawbacks. For example, a square wave may include frequency components above the Nyquist frequency that fold back into the signal when it is sampled by an analog to digital converter, and that may interfere with the gain measurements. In addition, the accuracy and stability of the circuit used to convert the digital square wave to a reference square wave signal having an amplitude referenced to a bandgap reference voltage may affect the measurement results.